In 1885 the Pennsylvania National Guard began a 50-year annual encampment at Mt. Gretna, using 120 acres of this area as its summer encampment site. This proved to be so satisfactory that it was 1935 before they moved to the Indiantown Gap area. That year Conewago Creek was dammed to form Lake Conewago, by the Army Corp of Engineers, ideal for swimming and canoeing.